If the program tries to do an offline activation, but you are sure you have an internet connection working, the most likely explanation is that a firewall is blocking online access to FSX, this means you have to turn it off, or configure it to grant online access to FSX.
Of course, nothing of this caused crashes to FSX, which were caused by something entirely unrelated to any of our products. Offline activation is a normal procedure and it can be used too, in case the server (which is not ours, it's Esellerate's and they have multiple redundant activation servers, so it's very unlikely they'll go down all at the same time)